1. Field of the Invention
This invention pertains to the field of urethane-modified polyisocyanurate rigid foams. More particularly, this invention pertains to the use of specific polyols utilized in preparing said foams.
2. Description of the Prior Art
Urethane-modified polyisocyanurate rigid foams are known in the art. Such foams are prepared by reacting a polyol, polyisocyanate and optionally other ingredients in the presence of a blowing agent. An isocyanurate group formation catalyst is used to trimerize the isocyanate groups to form the isocyanurate linkages. The polyol essentially acts as a modifying or reactive plasticizing agent in the overall polymeric scheme since a polymer containing only isocyanurate groups is itself too friable. Thus, the isocyanurate foam contains both isocyanurate groups, as well as urethane linkages, with said urethane linkages acting to plasticize the foam. Initially, the reaction proceeds to give a urethane quasi-prepolymer containing active isocyanate groups which during the subsequent reaction time, trimerize to give A polymer rich in isocyanurate linkages. This sequence ultimately produces a urethane-modified polyisocyanurate polymer.
Some main uses of the resultant foam include those of thermal insulation, and as building materials and the like. Examples of some prior art, isocyanurate foams and methods of preparation are described in U.S. Pat. Nos. 3,745,133; 3,644,232; 3,676,380; 3,168,483; and 3,519,950 to name a few.
However, many known polyisocyanurate foams have one or more disadvantages. In particular, rigid foams of this type often have high friability or propensity to break. Efforts to reduce friability have often resulted in sacrifice of dimensional stability, and thermal stability and flammability resistance. Lack of flammability resistance is particularly characterized by flame spreadability. Yet other prior art polyisocyanurate foams have suffered from disadvantages of low adhesiveness, irregular cell structure and the like.
One class of polyols disclosed as being useful in preparing polyisocyanurates are novolak resins or derivatives of novolak resins, including, alkoxylated novolak resins. See, for example, U.S. Pat. Nos. 3,723,364; 3,723,367; 3,728,293; 3,745,133; 3,842,036; and 3,849,349. In each instance, the novolak resin is prepared by reacting an excess of phenolic compound such as phenol itself with an aldehyde such as formaldehyde. The excess aromatic phenol is then removed and the resin used as such or derivatized, such as by preparing an oxyalkylated phenol-aldehyde resin. However, it has been found here that such novolak polyols, while useful in preparing rigid polyisocyanurate foams, nevertheless, still do not have the requisite degree of low friability necessary for a commercial application.
We have now found that it is now possible to prepare modified polyisocyanurate foams involving use of specific novolak polyols that do not exhibit any of the aforesaid disadvantages. We have particularly found that urethane-modified polyisocyanurate foams can be prepared having suitable dimensional stability, low friability and good flammability resistance. Such are achieved without sacrifice of other sought-after properties such as thermal stability.